Address decoding
The A14 and A15 address lines plus WR/ are attached to U506 chip select decoder. I'm assuming the "real" part number is 74xx139.
Based on this the first section's input pins 1G/ (pin 1), 1A (pin 2), and 1B (pin 3) decode as
- 1Y0 (pin 4) decodes 0x0000 -> 0x3fff (n/c)
- 1Y1 (pin 5) decodes 0x4000 -> 0x7ffff (n/c)
- 1Y2 (pin 6) decodes 0x8000 -> 0xbfff - CS/ to U502 (EEPROM)
- 1Y3 (pin 7) decodes 0xc000 -> 0xffff - CS/ to U501 (EPROM)
The second section's input pins 2G/ (pin 15), 2A (pin 14), and 2B (pin 13) decode as
- 2Y0 (pin 12) decodes 0x0000 -> 0x3fff (n/c)
- 2Y1 (pin 11) decodes 0x4000 -> 0x7ffff - CE/ to U503 latch
- 2Y2 (pin 10) decodes 0x8000 -> 0xbfff - (n/c)
- 2Y3 (pin 9) decodes 0xc000 -> 0xffff - CE/ to U504 latch
The "1" side is always active, as pin 1 is tied to ground. The "2" side only does something interesting on writes.
The U503 and U504 latches appear to be 74x373.